Digital signals, including digital differential signals, are used for signal transmission on circuit boards, integrated circuit packages, interposer substrates and motherboards to help protect a signal from picking up external noise. Digital differential signals are also used in computer systems and communication systems such as a local area network (LAN) system. A digital differential signal has two components that are 180 degrees out-of-phase with each other. The signal components transition between digital values of zero and one which may be represented by particular voltages. Digital differential signals are conventionally transmitted using a pair of traces spaced closely together on a circuit board or substrate. Each trace of the pair carries one of the components of the digital differential signal. FIG. 1 illustrates a cross-section of a portion of a circuit board with a pair of traces for communicating digital differential signals in accordance with the prior art. Circuit board 100 has trace pair 102 on insulating substrate 104. Conductive ground plane 106 is on a side of substrate 104 opposite to traces 102. Trace pair 102 illustrates a pair of closely spaced traces that have been conventionally used to carry digital differential signals. Because the signal components are out-of-phase, coupling between the traces reduces the signal's susceptibility to external noise. The darkened/solid areas (e.g., trace pairs 102 and ground plane 106) illustrated in the informal drawings are conductive material while the non-darkened areas may be a non-conductive or insulating material.
Semiconductor devices, computers, and other elements in digital systems continue to increase their operating data rate requiring the communication of digital differential signals of increasingly higher transition-rates. The transition-rate refers to the rate at which a digital signal transitions between states. In the near future, digital devices may require high-speed input/output (I/O) communications using digital differential signals, which may exceed rates of one giga-transition per second, and even rates of even ten giga-transition per second. One problem with conventional circuit board trace pairs is that as the transition-rate increases, conduction loss also increases. Furthermore, as the transition-rate increases, signal integrity degrades. Conventional circuit board trace pairs for carrying digital differential signals may have upper limits of less than one or ten giga-transitions per second and are therefore generally unsuitable for carrying higher rate digital differential signals. Digital differential signals above one and especially above ten giga-transitions per second when communicated over conventional trace pairs results in unacceptably high conduction loss and reduction in signal quality and integrity.
One technique that has been used to help reduce the increase in conduction loss associated with high transition-rate digital differential signals is increasing the trace width. Increased trace widths, however, reduce signal routing ability and consume more area on a circuit board, substrate or package, for example. Increased trace width also results in higher dielectric loss due to increased capacitance, which contributes to the reduction in signal quality and integrity.
Thus there is a need for improved communication of high-speed digital differential signals. There is also a need for a circuit board having traces suitable for communication of high-speed digital differential signals. There is also a need for traces for communicating high-speed digital differential signals with reduced conduction loss. There is also a need for traces for communicating high-speed digital differential signals with reduced dielectric loss. There is also a need for traces for communicating high-speed digital differential signals that help maintain signal integrity.
The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner.